1. Field of the Invention
The present invention relates to a powered steering mechanism for a vehicle and, in particular but not exclusively, to a powered steering mechanism for one or more secondary axles of a vehicle, which may comprise a motor vehicle or trailer.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
Conventionally, a vehicle with two or more axles will have its front or primary axle steered. Its other secondary axle or axles need not have any steering mechanism even though one or more of them may be driven. However, it can be an advantage if one or more of the secondary axles are also steered, for example, to increase maneuverability of the vehicle, to improve wheel tracking, to improve the vehicle's fuel economy and to reduce tire wear. Also, it may be advantageous to have the means to disable the steering of any secondary axle and, at the same time, to center the wheels to a neutral, straight-ahead position, and to maintain them in this position. The steering of secondary axles is usually controlled by the primary steering mechanism so that any movement of the primary steering mechanism results in an equivalent, proportional movement of the secondary steering mechanism. Control can be achieved by a mechanical linkage but this is heavy and bulky, and a centering arrangement for such a control would be difficult to arrange. Hence, normal conventional practice is to provide a power unit for steering secondary axles and to provide an auxiliary centering unit for maintaining the secondary axles in a neutral, straight-ahead position when required.
Conventionally, such a power unit takes the form of a hydraulic cylinder that is arranged between a fixed point on a vehicle's frame and a steering linkage. Within the cylinder, a piston reciprocates to provide the power assisted steering. The piston is connected to a rod that is connected to the steering linkage. Fluid pressure, on one or another side of the piston, as controlled by the valve unit, moves the rod and thereby the steering linkage in the required direction. The amount of fluid pressure applied to the piston by the valve unit depends on the force applied to the vehicle steering wheel by the driver.
The centering unit typically comprises either a pneumatic bellows which is separate from the power unit or an additional hydraulic cylinder that may or may not be attached to the steering cylinder to power the steering towards a central position. However, this may not necessarily power the steering into the actual central position, and separate mechanical means are often provided in addition to hold the unit in the central position once reached.
The object of the present invention is to simplify the aforementioned conventional arrangement with consequent savings in efficiency and cost and to provide a powered steering mechanism that combines the steering and centering mechanisms within the same cylinder.